This invention is concerned with a phase control device, and aims at attaining accurate phase control.
A conventional phase control device as illustrated diagrammatically in FIG. 1 of the accompanying drawing is of such a construction that a load 1 and a phase control element 2 such as a thyrister, TRIAC, and so forth are connected in series with an a.c. power source; a variable resistor VR and a time constant element of a capacitor C.sub.1, which are mutually connected in series, are connected in parallel with the abovementioned phase control element 2; a capacitor C.sub.2 is connected in parallel with the abovementioned capacitor C.sub.1 through a resistor R.sub.1 ; and a connection a of the resistor R.sub.1 and the capacitor C.sub.2 is further connected to a gate 2a of the phase control element 2 through a trigger element D such as DIALAC, etc..
This known phase control device is to change a time for an electric potential at the connection a to reach a breakdown voltage of the trigger element D by changing the CR time constant through variations in the resistance value of the variable resistor VR, thereby controlling a conduction period of the phase control element 2, i.e., a phase control angle.
However, this known device has such a disadvantage that, when a multitude of phase control circuits as shown in FIG. 1 are to be controlled simultaneously, the trigger element D in each phase control circuit is difficult to have a uniform characteristic, i.e., a uniform phase control angle, because of fluctuation in the breakdown voltage and of dependency of the trigger element on variations in an ambient temperature and a power source voltage to be supplied to it. Further, when the trigger element D is exchanged, the phase control angle varies due to fluctuation in the characteristics of the trigger element with the consequence that no accurate phase control can be achieved. In case the power source voltage is instable or a load is low, there tends to occur erroneous operation in the phase control.